1. Field of the Invention
The present invention relates to a head end of an optical fiber network that carries multiple optical signals in corresponding channels using a wavelength division multiplex channel structure. In particular, the invention relates to an apparatus and method for loading unutilized channels with noise so that information signals on utilized channels will not draw all of the power from optically pumped fiber amplifiers in repeaters of the optical fiber network.
2. Description of Related Art
Undersea communication network systems require repeaters periodically spaced to compensate for attenuation in the signal transmission medium. Optical fiber networks include repeaters connected between links of optical fiber cable. The optical fiber cables include one or more optical fibers and often include conductive wires (e.g., copper wires) to deliver power to the repeaters.
In order to maximize the transmission capacity of an optical fiber network, a single fiber is used to carry multiple optical signals in what is called a wavelength division multiplex system (hereinafter a WDM system). For example, a single optical fiber might carry 32 individual optical signals at corresponding wavelengths evenly spread between 1541 and 1589 nanometers (e.g., spread in channels on 1.5 nanometer centers). For example, a first information signal may be formed using on-off keying (OOK) of an optical signal at a wavelength of 1542.5 nanometers; a second information signal may be formed using on-off keying of an optical signal at a wavelength of 1544 nanometers; a third information signal may be formed using on-off keying of an optical signal at a wavelength system, a signal fiber will carry thirty-two (32) information signals spread in wavelength over the band between 1541 and 1589 nanometers.
Modern undersea WDM systems have a high traffic capacity, for example, a capacity to carry 32 channels of 10 gigabits per second (hereinafter Gb/s). When an undersea optical link is initially deployed, the link may be only partially loaded. Initially only a few of the 32 potential channels may be used to carry information signals of 10 Gb/s.
Repeaters as used in WDM systems that are deployed under the sea cannot easily be modified, and the repeater must be sized initially to support a fully loaded link (e.g., 32 channels, each channel carrying 10 Gb/s). A representative undersea repeater might be designed to provide an optical output signal at a nominal optical power of, for example, 32 milliwatts. The nominal output power level is insensitive to the power at the input of the amplifier. As the input power varies over a wide range, the output power changes very little around this nominal output power level. Thus, when the optical link is fully loaded with 32 channels, each channel will be amplified in the repeater to an optical output power of one (1.0) milliwatt per channel. However, if the initially deployed system uses only two channels for information, information signals on these two channels will draw all of the power from the optically pumped fiber amplifier, and the repeater will provided an output signal power of 16 milliwatts for each of the two channels (i.e., half of the 32 milliwatt output power of the repeater). As additional channels are added, the optical output power per channel will become reduce from 16 milliwatts to 1.0 milliwatts when the fiber link is fully loaded.
In a fiber optic network, the fiber medium itself is non-linear. This nonlinearity interacts with the dispersion of the fiber, and degrades the network performance. At high optical powers (e.g., more than 10 milliwatts per channel), the optical signal experiences more distortion than at low optical powers (e.g., less than 1.0 milliwatt per channel). Since the in-line repeaters of the network that are deployed undersea have a substantially constant output power level (e.g., total power of all channels), the optical power per channel at initial deployment is much higher than the optical power per channel in a fully loaded optical network. As a result of the nonlinearity, the network communication performance at initial deployment could be worse than the performance when the network is fully loaded.